Portable radios are often used in conjunction with accessories which allow certain radio features to be enabled remotely. For example, remote speaker microphones are often worn by police officers in order to free up their hands and still maintain easy access to the radios' main functions. An operating feature which is typically enabled remotely is the push-to-talk (PTT) option which allows a user to push a PTT button on the remote accessory to talk through an external microphone, and release it to listen through an external speaker. Other features such as volume control and microphone gain can also be set by the user at the remote accessory. FIG. 1 of the invention shows a basic block diagram of a radio 102, and its remote enabled accessory 104 including speaker 106 and microphone 108. Remotely enabled accessories are typically interfaced to the radio through input/output (I/O) ports 110 located on both the radio 102 and the accessory 104.
Option select schemes of the past have typically used multiple I/O ports to transfer the various radio functions remotely. The addition of I/O ports between the radio and its remote accessory usually entails additional desense and static protection circuitry causing an increase in the size and cost of both the radio and the accessory. Also, if the accessory is coupled to the radio through a cable the increase in I/O ports translates into a thicker cable which has the disadvantage of adding bulk and weight for the user.
Accordingly, there is a need for an option select scheme that reduces the number of I/O ports between a radio and its' accessory.